Hydrocarbon synthesis, also known as Fischer-Tropsch, processes involve the catalytic hydrogenation of synthesis gas, hydrogen and carbon monoxide, to form higher hydrocarbons, e.g., C.sub.2 +, but preferably C.sub.5 +materials. The synthesis gas feed can also contain small amounts of CH.sub.4 and CO.sub.2 as carry overs from the synthesis gas manufacturing step.
The hydrocarbon synthesis reaction produces hydrocarbons and water, and at reaction conditions the water product can readily react with CO in the water gas shift reaction EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2
to produce CO.sub.2, a deleterious side reaction since the objective of the process is the production of higher hydrocarbons from H.sub.2 and CO. By the use of so-called "non-shifting" catalysts, e.g., cobalt on titania, silica, or alumina which have low selectivity for CO.sub.2 ; the water gas shift reaction is largely suppressed.
Iron based hydrocarbon synthesis catalysts usually promote the water gas shift reaction and have a high selectivity to CO.sub.2. Thus, by virtue of the relatively high CO concentration in the synthesis gas feed and the fact that water is produced in the synthesis process as hydrogen is reacted, the water gas shift reaction becomes important. The shift reaction can, however, be suppressed if the feed contains higher amounts of CO.sub.2 relative to CO, and CO.sub.2 is known to be added to H.sub.2 +CO synthesis gas feeds for hydrocarbon synthesis. Synthesis gas feeds can contain up to 10% CO.sub.2, that is, about 0.1-10% CO.sub.2.
The invention described herein makes use of non-shifting catalysts to convert a large portion of the CO in the synthesis gas to desired hydrocarbons, resulting in an enrichment of the gas phase CO.sub.2 concentration, followed by the use of a shifting catalyst wherein the presence of CO.sub.2 suppresses further shifting and results in exceedingly high overall CO conversions in the order of 95+%, preferably 97% or greater. These CO conversion levels substantially eliminate the need for any recycling of hydrocarbon synthesis products back to the hydrocarbon synthesis reactors, thereby eliminating the need for costly recompression and treatment of recycled gases for removing oxygenated products, e.g., alcohols, aldehydes, that may poison or suppress the synthesis reaction.